1) Field of the Invention
The present invention relates to an amorphous metal-metal composite article, a method for producing the same, and a torque sensor using the same. More particularly, the present invention relates to an amorphous metal-metal composite article, comprising an ordinary crystallized metal and an amorphous material (hereinafter may be referred to as amorphous), such as amorphous metal or amorphous alloy, which has been produced from its melted state through rapid cooling and has no crystal structure, firmly bonded to the crystallized metal by the explosion pressure of an explosive. The present invention also relates to a method for producing the composite article, and a torque sensor using the same.
Amorphous metal has excellent magnetic properties, and hence attempts have been made to use the metal as various magnetic materials. Particularly, a composite article comprising an amorphous metal and an ordinary metal bonded thereto is very useful as a torque sensor due to the soft magnetic property and high magnetostriction of the amorphous metal.
2) Related Art Statement
Amorphous can be produced by a rapid cooling method or other various methods, such as spatter method, chemical vapor deposit method, plating method and the like. However, the amorphous metals obtained by these methods are thin sheets, fine wires and powders, all of which have a dimension of less than several hundreds .mu.m, and the use field of these amorphous metals is very limited. In order to use amorphous metals in a wide field, amorphous metals having a larger dimension are demanded, and a shaped article produced by the compression molding of amorphous metal powder has been disclosed (Japanese Patent Laid-open Specification No. 61-139,629).
Japanese Patent Laid-open Specification No. 61-195,905 discloses an amorphous-covered metal obtained by a method wherein an amorphous metal powder is bonded to a metal matrix by an explosion pressure; and Japanese Patent Laid-open Specification No. 62-23,905 discloses a method for producing a composite sintered article from an amorphous metal powder and a metal powder by utilizing a high energy shock.
There has been proposed a technic, wherein an amorphous metal thin sheet having a thickness of not larger than 100 .mu.m is bonded to a sheet or round rod of metal by an explosion pressure to produce a composite article having both of the excellent magnetic properties inherent to amorphous metal and the high strength inherent to metal. However, amorphous metal is generally lower in thermal expansion coefficient than ordinary metal. Therefore, when a composite article comprising an amorphous metal and a metal bonded to the amorphous metal through the explosion bonding, which metal is higher in thermal expansion coefficient than the amorphous metal, is heat treated in order to restore the magnetic properties deteriorated due to the residual stress caused in the amorphous metal during the explosion treatment, a compression stress is caused in the amorphous metal during the cooling of the composite article from the heat treatment temperature to room temperature. This compressor stress is due to the reason that the metal bonded to the amorphous metal shrinks more than the amorphous metal bonded with the metal shrinks, and hence magnetic domains extending in a direction perpendicular to the length direction of the composite article are formed, and a satisfactorily excellent magnetic property can not be obtained in the length direction of the composite article. In order to prevent these drawbacks, it is necessary that an amorphous metal and a metal having substantially the same thermal expansion coefficient are bonded to each other so as not to cause a compression stress in the amorphous metal. (Magnetic Society of Japan, 10th Autumn Annual Meeting Preprint No. 4PC-10 (page 61) (1986, 11), Ichiro Sasada et al, "Properties of Torque Sensor produced by the Explosion Bonding Method").
The inventors have attempted to develop a composite article comprising an amorphous metal and substantially any metal bonded thereto and being able to be used as a magnetic sensor, such as a torque sensor or the like, by utilizing the excellent magnetic properties inherent to the amorphous metal, and have studied the above described problems and further made various theoretical and experimental investigations.
In general, amorphous metal causes a structural relaxation by a heat treatment and exhibits improved magnetic properties after a heat treatment as compared with the magnetic properties just after the production. Therefore, amorphous metal is generally used after it has been subjected to a heat treatment. However, when an amorphous metal-metal composite article, wherein the amorphous metal has been very tightly bonded to the metal by the explosion bonding, is subjected to a heat treatment, the magnetic properties inherent to the amorphous metal are often deteriorated without being restored to the original values before the heat treatment.
The above described prior art discloses the methods for solving this problem. However, the prior art has still the following drawbacks and has been difficult to be carried out.
The reason is probably as follows.
(1) When an amorphous metal thin sheet is merely applied with an explosion pressure without bonding to a metal by the explosion, and it is intended to restore the deteriorated magnetostriction of the amorphous metal thin sheet by removing the residual stress, which is caused in the amorphous metal thin sheet by the explosion, by a heat treatment, the deteriorated magnetostriction of the amorphous metal thin sheet can be restored to about 90-100% of its original value by keeping the sheet for several hours at a temperature of 250.degree.-400.degree. C. to remove the residual stress.
(2) However, in the case where a composite article comprising an amorphous metal thin sheet and a metal bonded thereto by the explosion is heat treated, when the length of both the elements of the composite article, after heated up to the heat treatment temperature, is represented by L, the difference between the room temperature and the heat treatment temperature is represented by T, and the thermal expansion coefficients of the amorphous metal and the metal bonded with each other are represented by .alpha. and .alpha.' respectively, the lengths of the amorphous metal and the metal after cooled to room temperature after completion of the heat treatment can be represented by L(1-.alpha.T) and L(1-.alpha.'T), respectively.
(3) When the metal bonded to the amorphous metal has a thermal expansion coefficient higher than that of the amorphous metal, {L(1-.alpha.T)} is larger than {L(1-.alpha.'T)}.
(4) An amorphous metal thin sheet is ordinarily bonded to a metal having a thickness larger than the thickness of the amorphous metal thin sheet, and therefore the amorphous metal thin sheet is compressed by the shrunk metal.
(5) An amorphous metal forms magnetic domains extending in a direction perpendicular to its length direction when it is compressed, and hence the magnetic properties (magnetization) of the amorphous metal in its length direction are noticeably deteriorated, and the composite article can not be practically used.
In order to solve the above described problems, the following means is considered to be effective.
(1) There are used an amorphous metal and a metal having substantially the same linear thermal expansion coefficient.
(2) The heat treated composite article is subjected to a plastic working which can apply a tensile stress to the amorphous metal in an amount sufficient to offset the compression stress caused in the amorphous metal during the heat treatment.
The above described means (1) has drawbacks explained in the following items (a) and (b), and is not suitable to be carried out in a commercial scale.
(a) The means (1) requires merely to select the kind of metals to be bonded to amorphous metal, and is simple. However, in the practical operation, these metals are Zr, Ti, Mo, Ta and their alloys containing other alloy elements, and particular alloys. These metals and alloys have drawbacks that, for example, they are difficult to be obtained, or are insufficient in the strength to be used as a sensor, or are difficult to be bonded to amorphous metal.
(b) When a composite article is to be worked into, for example, a torque sensor, it is necessary that an amorphous metal thin sheet is bonded to a metal round rod, and the resulting composite article is worked into a torque sensor, or that a composite article obtained by bonding an amorphous metal thin sheet to a metal flat sheet is wound round and bonded to a round rod used as a reinforcing member. In the former case, there is a problem that the working is difficult in addition to the defects explained in item (a). In the latter case, it is necessary to carry out the bonding at a temperature, which does not cause crystallization of the amorphous metal. Therefore, it is difficult to carry out the bonding of the composite article to the reinforcing round rod by the welding, press bonding, soldering and the like, in which the amorphous metal is exposed to a high temperature. As an easy method for the bonding, there can be used a mechanical bonding of the composite article to the reinforcing round rod, or an adhesion of the composite article to the reinforcing round rod by means of an adhesive. In the mechanical bonding, a composite article (hereinafter, referred to as sensor) comprising an amorphous metal and a metal bonded thereto is bonded to a round rod (hereinafter referred to as shaft) by means of a mechanical screw, or by a mechanical fitting or the like. In this case, there is a problem that stress is unevenly transmitted from the shaft to the sensor, or other problem. When the sensor is adhered to the shaft by means of an adhesive, there are such problems that the adhesive is only durable in a very narrow range of the environmental conditions, under which the torque sensor is used, and that it is difficult to maintain the adhered state for a long period of time of the use. Moreover, in the case where the flat sheet-shaped sensor is wound round a shaft, it is difficult to bond the flat sheet-shaped sensor to the shaft in a high roundness, and hence when the resulting sensor assembly formed of the sensor and the shaft is used as a torque sensor, the measured values are dispersed due to the following reasons unless a large gap is formed between the sensor and a pick-up used for measuring the variation of magnetic flux, both the members come into contact with each other; the sensitivity of the sensor is deteriorated by the formation of a large gap; and the distance between the pickup and the sensor varies, or other reasons.
Therefore, the inventors considered that the means (2) is a proper means for bonding the sensor to the shaft, and have made various theoretical and experimental investigations with respect to various methods for bonding the sensor to the shaft by considering that the main object of the present invention is the provision of a sensor, such as torque sensor or the like, which utilizes excellent magnetic properties inherent to amorphous metal. As a result, the inventors have accomplished the present invention. That is, according to the present invention, magnetic sensors, such as torque sensor and the like, having excellent magnetic properties and high mechanical accuracy, and hence having high sensitivity at the use and small dispersion in the measured values can be obtained even when an amorphous metal is bonded to a metal having substantially any thermal expansion coefficient.